Rabbit immunoglobulin allotypes are antigenic determinants thought to be inherited as autosomal co-dominant alleles. This notion has been challenged by observations of low concentrations of allotypes not detected by qualitative tests or predicted by parental genotypes. In the past year, L chains with "latent" allotypes were isolated from the sera of pedigreed rabbits and shown by amino acid sequence analysis to be indistinguishable from the normal allotypes. Current research focus on hybridoma cells maintained in culture that synthesized rabbit immunoglobulin RNA will be isolated from these cell lines and cDNA probes will be prepared to carry out molecular biological studies in order to determine the basis for inheritance of latent allotypes. Other studies in this area have included clearance of normal versus latent allotypes in animals using doubly labelled material and studies on the linked expression of VH markers on immunoglobulins expressing latent allotypes.